Method and apparatus for automatically controlling purge rates



H. w. HUSA 3,177,385

METHOD AND APPARATUS FOR AUTOMATICALLY CONTROLLING PURGE RATES A ril 13, 1965 2 Sheets-Sheet 1 Filed May 25, 1962 Fig. 1

DIFFERENT/AL THERMOGOUPL E cow/mum i Ixi r w Purge Gas p y V TEMPERATURE DlFFERENTlAL, F

l NVENT OR. Howard William l-lusa ATTORNEY H. W. HUSA April 13, 1965 METHOD AND APPARATUS FOR AUTOMATICALLY CONTROLLING PURGE RATES 2 Sheets-Sheet 2 Filed May 25. 1962 Fig. 2

CONTROLLER Purge Gas pp y ATTORNEY United States Patent Brad 3,177,885 METHOD AND APPARATUS FOR AUTGMATI- CALLY CONTROLLING PURGE RATE Howard William Husa, Park Forest, Ill, assignor to tandard Oil Company, Chicago, 111., a corporation of Indiana Filed May 25, 1962, Ser. No. 197,760 8 Claims. (Cl. 137-3) This invention relates to a method and apparatus for automatically controlling the flow of a condensable purge gas to a vessel, such as a stack, tank, drum or line. It is particularly related to an improved method and apparatus for controlling the concentration of combustion support gases within a flare, vent or blow-down stack. More specifically, the invention concerns apparatus for automatically regulating the flow of purge gas to a stack.

In industry, and particularly in the petroleum refining industry, it is often necessary to remove combustible gases or combustiton support gsaes from a vessel such as a stack, tank, drum or line. This may be accomplished by purging with a gas such as steam, nitrogen, carbon dioxide or refinery fuel gases. The purge gas replaces the undesirable combustion support gas and renders the stack nonflammable to combustibles present or subsequently introduced. Another function of purging with steam or a hot purge gas is to supply heat to the vessel to oiiset heat transfer losses.

To maintain a stack free from combustion support gases, at purge gas is introduced near the base of the stack. A lighter-than-air purge gas such as nitrogen or refinery fuel gas will tend to rise rapidly up the stack. The velocity of the ascending purge gas causes turbulence and mixing of the purge gas with the gases attempting to diiluse down the stack so as to flush the stack free of air and other combustion support gases. Purge gases heavier than air displace air and other gases attempting to diffuse down the stack, in addition to exerting a flushing action. Thus,

the re-entry of air into a stack may be averted or controlled by continuouslyintroducing a purge gas into the stack. By controlling the rate of re-entry of air through the top of the stack, a controlled, noncombustible oxygen concentration may be maintained Within the stack. Similar procedures can be carried out on other vessels, such as tanks, drums and lines. In each case the operation of the purge is similar.

It has heretofore been considered safe practice to introduce a purge gas, such as steam, to a vessel, such as a stack, at a rate sufiicient to create a visible plume at the top of the stack. The presence of the Visible plume was believed to insure the absence of air in quantities sufiicient to support combustion in critical parts of the stack. It has been found that the presence of the visible plume does not necessarily render the stack nonflammable by preventing the presence of combustion support gases within critical parts of the stack. Tests have shown that a visible plume can exist with up to 50 percent air concentration within the stack. It was also customary in the prior art to maintain the purge rate essentially constant once the visible plume was established. It has been found that purging at a relatively constant rate without compensating for atmospheric conditions may Waste purge gas on warm days and may permit the accumulation of explosive gas mixtures within the stack on cold days.

It is an object of this invention to provide a method and apparatus for automatically insuring a purge rate sufiicient to preclude combustible gas mixtures from critical parts of vessels such as stacks, tanks, drums and lines. Another object is to provide a method and apparatus for automatically controlling the flow of a condensable purge gas to vessels of the said type. It is also an object of this invention to provide a method and apparatus for automatically optimizing the purge rate in relation to atmospheric conditions, thereby avoiding the waste of purge gas resulting from purging at a constant rate. A more particular object is a method and apparatus for automatically maintaining safe concentrations'of air Within critical parts of stacks. Other objects will become obvious to those skilled in the art on reading the detailed description of my invention. V

I do not choose to be limited by the theories of my invention, but the invention is understood to operate in the following manner. In a vessel, such as a stack, tank, drum or line, a substantially isothermal zone and a declining temperature zone are established by introducing to the vessel a condensable purge gas. It is believed that conditions within the vessel to be purged must be such that a partial condensation of the purge gas can occur when the purge gas is introduced to the vessel. The isothermal zone is thus understood to result from the release of heat of condensation from the condensable purge gas and will contain substantially no non-condensable gas.

The geometry of the isothermal zone within the vessel will depend upon the type vessel, external air temperatures, rain, wind velocity, purge gas temperature and purge rate. Experimental measurements have indicated substantially no non-condensable gas is present in the isothermal zone. No significant quantities of non-codensable gas, such as air, enter the vessel other than through the vent or open top. From the end of the isothermal zone to the vent of the vessel, the temperature continuously declines and the non-condensable gas increases proportionately as the differential between the isothermal temperature and the declining temperature increases.- I have discovered the concentration of non-condensable gases, such as air, oxygen or nitrogen, at a selected point within the declining temperature zone of the vessel may be correlated to the temperature difierential between the selected point in the declining temperature zone and the isothermal temperature. The present invention employs this relationship between differential temperatures and non-condensable gas concentration to automatically control the purge rate and maintain the vessel safe from accumulation of non-condensable gases, such as combustion support gases.

Broadly, there is provided a method for controlling the concentration of non-condensable gases, such as nitrogen, hydrogen or combustion support gases within vessels, such as stacks, tanks, drums and lines wherein a condensable purge gas establishes a substantially isothermal zone and a declining temperature zone in the vessel. The method comprises introducing a condensable purge gas to the vessel and controlling the flow of purge gas to the vessel to maintain a preselected temperature differential between a selected point in the declining temperature zone and the isothermal temperature whereby the non-condensable gas concentration at the selected point in the declining temperature zone is maintained at a predetermined value. As apparatus for efiecting the control of combustion support gases within vessels, suchas stacks, tanks, drums and lines, there is provided two heat sensitive means, one to be placed in the substantially isothermal zone of the vessel and the other in the declining temperature zone. A valve disposed in the purge gas supply line and a controller responsive to temperature variations at the heat sensitive means and capable of actuating the valve disposed in the purge gas supply line vary the purge rate in response to diiferential temperature variations between the heat sensitive means.

In a preferred embodiment of the present invention for controlling purge rates to stacks, the apparatus of the invention includes a differential thermocouple, one terminal to be placed in the substantially isothermal zone of a stack and the second terminal to be placed at a see3 lected elevation in the declining temperature zone of the stack. In the purge gas supply line there is a valve. The valve is preferably a pneumatic control valve, although it could be electrically or mechanically controlled. Pneumatic valves capable of regulating the purge gas rate are well known and commercially obtainable. The leads from the terminal of the differential thermocouple pass to a controller. The controller is responsive to variations in differential temperaturebetween the thermocouple terminals and capable of actuating the purge gas supply line valve in response to such differential temperature changes. Controllers of this general type are commercially available and their construction does not constitute a part of the present invention. Examples of commercial controllers which are suitable for use in the present invention are a Honeywell ElectroniK Neumatic Controller and a Foxoboro Dynatrol M-40 Pneumatic Controller.

'In a preferred method steam is introduced to a stack through a supply line to purge the stack of air, and there is established therein an isothermal zone and a declining temperature zone. Heat sensing means are utilized to sense the isothermal zone temperature and the temperature at a selected point in the declining temperature zone of the stack. Impulses from the heat sensitive means are passed to acontroller. The controller is set to continuously maintain a preselecteddifterential temperature between the isothermal temperature and the temperature at a selected point in the declining temperature zone by actuating the valve in the steam supply line. The air concentrationat the selected point in the declining temperature zone is directly related to the temperature differential between that point and the isothermal temperature.

Thus, by varying the steam flow rate to maintain the preselected temperature differential, the air concentration at the selected point in the declining temperature zone is continuously controlled.

To further illustrate the present invention, reference is made to the drawings.

FIGURE 1 illustrates a preferred embodiment and schematically shows the apparatus of the present invention in relation to a stack.

FIGURE 2 shows the apparatus of the invention as employed on a vessel.

FIGURE 3 is a plot of oxygen concentration at the se-- lected point in the declining temperature zone vs. temperature differential between that point and the isothermal temperature for a stack wherein the heat sensing means is positioned axially.

In FIGURE 1 a purge gas input line 1 supplies a purge gas, such as steam, nitrogen, or refinery fuel gases to a stack 2. A valve 3 is positioned in the purge gas input line 1 and 'operatively connected to the controller 4' through the linking means 6. The valve 3 may be pneumatically, electrically or mechanically actuated by the controller 4 and the character of the linking means 6 will depend on the type valve employed. Preferably, the valve 3 is a pneumatically actuated valve and the controller 4 is an electronic-pneumatic controller operating the valve 3 through a pneumatic pressure line 6. Leads 12 and 13 connect the differential thermocouple terminals 7 and 8 to the controller 4. The lower thermocouple terminal 7 is positioned axially in the isothermal zone 9,

preferably near the base of the stack 2. Terminal 8 of the differentialthermocouple is positioned axially in the eclining temperature zone 11 of the stack 2.

Referring to FIGURE 2, the vessel 14 may be a reaction vessel, tank, drum or line; The vessel 14 may be at atmospheric pressure, sub-atmospheric or super-atmospheric pressure. The outlet 16 may vent to the atmosphere or into a pressurized system and the inlet. 17 may connect the vessel 14 into a supply line 15. For the apparatus of this invention to operate .on such vessels when header'lfi does not open to the atmosphere, it is 4 necessary that there be a sufficient pressure drop across the vessel to permit purge gas to flow from the purge gas input line 18 out the header 1d. The valve 3a is posttioned in the purge gas input line 18 and a purge gas, such as steam, nitrogen, or refinery fuel gas is introduced to the vessel 14. The controller 4:: is operatively connected to the valve 3:: by the linking means 6a and actuates the valve 3a to vary the purge gas input in response to impulses from the heat sensing means 21 and 22. Leads 19 and 2% connect the controller to the heat sensing means 21 and 22 which are positioned on the wall of the essel 14.

FIGURE 3 graphically illustrates the relationship between non-condensable gas concentration, in this case oxygen, at a selected point in the declining temperature zone and the differential temperature between that point and the isothermal zone temperature. The relationship illustrated by FIGURE 3 is for stacks and vessels at atmospheric pressure where the heat sensing means are positioned axially to permit purge gas vapors to directly contact the heat sensing means. Under these conditions, as illustrated by the graph, the non-condensable gas concentration at any point within the declining temperature zone is constant for a given temperature differential between that point and the isothermal temperature.

Now to illustrate the operation of the present invention through a specific non-limiting example. It is desirable to establish and maintain within tall stacks an oxygen concentration of not greater than 6 percent at an elevation of percent of stack height. A 6 percent oxygen concentration is non-flammable to most hydrocarbon combustibles and flammable mixtures above the elevation of 75 percent of stack height are not generally hazardous. 7 To establish and maintain this maximum 6 percent oxygen concentration, the upper thermocouple terminal is placed axially within the stack at 75 percent of stack height. The lower thermocouple is placed near the base of the stack. It has been found that a differential temperature of. 17 F. between the upper thermocouple and an isothermal zone temperature which is established by the introduction of steam will result in a 6 percent oxygen concentration at the elevation of the upper thermocouple. Steam is introduced to the stack through a purge gas supply line which has a pneumatically actuated valve. A controller operates the valve in response to differential temperature changes between the terminals of the thermocouple. The controller is set to maintain the desired 17 F. between the thermocouple terminals by varying the purge gas flow rate. When the temperature differential between the isothermal zone temperature and the temperature at the upper thermocouple in the declining temperature zone reaches 17 F., the oxygen concentration at the elevation of the upper thermocouple terminal will be 6 percent. As this 17 temperature differential tends to vary due to excessive purge rates or changes in weather conditions, the thermocouple will sense the temperature variation and the controller will actuate the purge rate control valve to continuously re-establish the 17 temperature differential. Thus, a 6 percent oxygen concentration at 75 percent of stack height is established and continuously maintained under fluctuations of weather conditions and steam delivery conditions.

The present invention possesses broad applicability. One of the more obvious applications of the invention is to steam purge a vessel or stack of a gas which is not normally condensable at atmospheric conditions, such as air. However, purge gases other than steam may be used under temperature and pressure conditions which permit at least a portion of the purge gas to condense when introduced to the vessel to be purged. Neither is the invention limited to purging air and combustion support gases, for it may be used to purge hydrogen or nitrogen. When using a condensable purge gas at temperatures and pressures other than atmospheric, the relationship between temperature differential and non-condensable gas concentration will vary. However, the relationship may be ex perimentally determined for any system. To determine the relationship a condensable purge gas is introduced to the vessel, and an isothermal zone and a declining temperature zone are established. At selected elevations in the declining temperature zone non-condensable gas concentration is measured and a temperature reading taken. A curve of non-condensable gas concentration vs. temperature differential will indicate the differential tempera ture which must be maintained between any point in the declining temperature zone and the isothermal zone temperature to produce the desired non-condensable gas concentration.

In applying the apparatus and method herein described to vessels, such as drums, tanks and lines, it is important to carefully note the geometry of the system. Frequently purge gas inlets are located at elevated points within the system and vents are located at a lower point. Here the thermal pattern will be inverse of that observed in a vent stack, such as that illustrated in FIGURE 1. In other vessels, such as knock-out drums, the purge gas inlet and outlet may be both in the top of the drum. The substantially isothermal zone would, therefore, exist at the top inlet of the drum and the declining temperature zone is at the bottom of the drum. Since the volume of non-condensable gas in such a system is finite, the non-condensable gas will be removed through mixing and diffusion into the condensable purge gas. Thus, the apparatus and method of this invention are applicable to vessels, such as knock-out drums.

When the thermocouple terminals are placed axially within a stack or vessel, the relationship between difierential temperature and non-condensable gas concentration at the terminal in the declining temperature zone has been found to be independent of vessel or stack diameter. In this preferred arrangement of the thermocouple terminals, the condensable purge gas directly contacts the ther mocouple terminal. An alternative method is to place the thermocouple terminals on the wall of the stack or vessel, thereby measuring the temperature of the wall. The relationship between differential temperature and non-condensable gas concentration at a point in the declining temperature zone is dependent upon vessel or stack diameter when the thermocouples are placed on the wall. For stacks of ins. to 24 ins. in diameter, which covers the range commonly used in the petroleum refining industry, the relationship between differential temperature and oxygen concentration when using steam as a purge gas has been found to vary from 17 to F. for 6 percent oxygen concentration. The exact temperature dif ferential required to produce a particular oxygen concentration at a point in the declining temperature zone may be determined by the experimental method described above.

The term condensable gas as used in this specification means a gas which is at least partially condensable under temperature and pressure conditions existing in the vessel or stack. For instance, steam is a condensable gas when introduced to a vessel or stack at atmospheric pressure and temperatures below 212 F. The term non-condensable gas thus means any gas which is not condensable under temperature and pressure conditions existing in the vessel or stack.

Having described my invention, I claim:

1. The method of purging a vessel of non-condensable gases which comprises: introducing to a vessel a condensable purge gas to establish within said vessel a substantially isothermal zone and a declining temperature zone; determining the temperature differential between said isothermal zone and a selected point in said declining temperature zone when the concentration of non-condensable gas at said selected point is at a desired value; and thereafter controlling the flow of said purge gas in response to said temperature difierential in such a manner as to maintain the temperature differential corresponding 6 to the desired non-condensable gas concentration at the selected point.

2. The method of purging a vessel of non-condensable gases which comprises: introducing to said vessel a condensable purge gas at temperature and pressure conditions which permit at least a portion of said purge gas to condense when introduced to said vessel and thereby establishing in said vessel asubstantially isothermal zone and a decliningtemperature zone; regulating the flow of purge gas to said vessel in response to the temperature differential between the isothermal zone temperature and the temperature at a selected point in the declining temperature zone, which temperature diiferential is correlated with the concentration of non-condensable gas at said selected point, to continuously control the non-condensable gas concentration at the selected point in the declining temperature zone.

3. The method of purging a stack of non-condensable gases which comprises: introducing to said stack a condensable purge gas at temperature and pressure conditions which permit at least a portion of said purge gas to condense when introduced into said stack and thereby establish in said stack a substantially isothermal zone and a declining temperature zone; determining the temperature differential between said isothermal zone and a selected point in said declining temperature zone; regulating the flow of purge gas to said stack to maintain a predetermined temperature difierential between said selected point in the declining temperature zone and the isothermal zone temperature to continuously maintain the non-condensable gas concentration at the selected point in the declining temperature zone at a preselected value corresponding to the differential temperature between the selected point in the declining temperature zone and the isothermal temperature. I

4. The method of purging a vessel open to the atmosphere of non-condensable gases which comprises: introducing to said vessel steam at temperature and pressure conditions which permit at least a portion of said steam to condense in said vessel and thereby establish in said vessel a substantially isothermal zone and a declining temperature zone; regulating the flow of steam to said vessel to maintain a predetermined temperature differential between the isothermal zone temperature and the temperature at a selected point in the declining temperature zone, to continuously maintain the non-condensable gas concentration at said selected point at a preselected value corresponding to said predetermined temperature differential.

5. The method of purging a stack of combustion support gases which comprises: introducing to said stack steam at temperature and pressure conditions which permit at least a portion of said steam to condense in said stack and thereby establish in said stack a substantially isothermal zone and a declining temperature zone; determining the temperature difierential between said isothermal zone and a selected point in said declining temperature zone when the concentration of combustion support gas at said point is at a desired value; and thereafter controlling the flow of said steam in response to said temperature diflerential in such a manner as to maintain the temperature difierential corresponding to the desired combustion support gas concentration at the selected point.

6. In the method of purging a vessel open to the atmosphere of non-condensable gases by introducing into said vessel a condensable purge gas at temperature and pressure conditions which permit at least a portion of said purge gas to condense when introduced to said vessel thereby establishing in said vessel a substantially isothermal zone and a declining temperature zone, the improvement which comprises: measuring the concentration of non-condensable gas at a selected point in said declining temperature zone, determining the temperature differential between said isothermal zone and said selected point when the concentration of non-condensable gas is at a U desired value, and thereafiter controlling the flow of said purge gas in response to the temperature difierential be tween the isothermal zone and said selected point in such a manner as to maintain the determined differential corresponding to the desired non-condensable gas concentration at the selected point.

7. In the method of purging a stack of non-condensable gases by introducing into said stack steam at ternperature and pressure conditions which permitat'least a portion of said steam to condense when introduced into said stack thereby establishing in said stack a substantially isothermal zone and a declining temperature zone, the improvement which comprises: regulating the flow of steam to said stack and measuring the concentration of non-condensable gas at a selected point in said declining temperature zone, maintaining the temperature difierential between the isothermal temperature and the temperature at said selected point in the declining temperature zone corresponding to the desired non-condensable gas concentration at the selected point, whereby the concentration of non-condensable gas at the desired point is continuously controlled at the desired value.

8. In combination with process means wherein combustible vent gas is exhausted to the atmosphere, a ventgas stack system for increasing safety and minimizing purge steam usage which comprises: a substantially vertical stack, conduit means for passing combustible vent gas from said process means into said stack, conduit means for introducing purge steam into said stack near the bottom thereof, valve means adapted to control the flow rate of said steam, first temperature sensing means adapted to sense the temperature at a point within the lower portion of said stack and above the point of introduction of said steam, second temperature. sensing means adapted to sense the temperature within the upper portien of said stack above said first means, and controller means adapted to adjust said valve means to control said flow rate of steam in response to the diiference between the temperatures sensed by said first and said second temperature sensing means. 7

References Cited in the file of this patent UNITED STATES PATENTS 491,975 Barnes Feb. 21, 1893 1,767,588 Hutton June 24, 1930 1,849,335 Schmidt Mar. 15, 1932 

1. THE METHOD OF PURGING A VESSEL OF NON-CONDENSABLE GASES WHICH COMPRISES: INTRODUCING TO A VESSEL A CONDENSABLE PURGE GAS TO ESTABLISH WITHIN SAID VESSEL A SUBSTANTIALLY ISOTHERMAL ZONE AND A DECLINING TEMPERATURE ZONE; DETERMINING THE TEMPERATURE DIFFERENTIAL BETWEEN SAID ISOTHERMAL ZONE AND A SELECTED POINT IN SAID DECLINING TEMPERATURE ZONE WHEN THE CONCENTRATION OF NON-CONDENSABLE GAS AT SAID SELECTED POINT IS AT A DESIRED VALUE; AND THEREAFTER CONTROLLING THE FLOW OF SAID PURGE GAS IN RE- 